A known on-board evaporative emission control system for a motor vehicle comprises a vapor collection canister that collects volatile fuel vapors generated in the headspace of a fuel tank by the volatilization of liquid fuel in the tank and a purge valve for periodically purging fuel vapors to an intake manifold of the engine. A known type of purge valve, sometimes called a canister purge solenoid (or CPS) valve, is under the control of a microprocessor-based engine management system, sometimes referred to by various names, such as an engine management computer or an engine electronic control unit.
During conditions conducive to purging, the purge valve is opened by a signal from the engine management computer in an amount that allows intake manifold vacuum to draw fuel vapors that are present in the tank headspace and/or stored in the canister for entrainment with combustible mixture passing into the engine's combustion chamber space at a rate consistent with engine operation so as to provide both acceptable vehicle driveability and an acceptable level of exhaust emissions.
Certain governmental regulations require that certain motor vehicles powered by internal combustion engines which operate on volatile fuels such as gasoline have evaporative emission control systems equipped with an on-board diagnostic capability for determining if a leak is present in the evaporative emission control space.
One known type of vapor leak detection system for determining integrity of vapor containment space, i.e. evaporative emission control space, performs a leak detection test by positively pressurizing the evaporative emission control space using a positive displacement diaphragm pump. Associated valves, such as the purge valve and any vent valves are closed, and the diaphragm pump is reciprocated to create test pressure. Commonly owned U.S. Pat. No. 6,192,743, issued Feb. 27, 2001, discloses a module comprising such a pump.
Known test methods include operating the pump to create superatmospheric pressure in the closed space being tested and then detecting changes that are indicative of leakage. One method comprises measuring a characteristic of pump operation. An example of a time-based measurement is a measurement of how frequently the diaphragm pump must be cycled in order to maintain pressure. Other methods of measurement are pressure-based, such as measuring the rate at which pressure decays.
Another known type of vapor leak detection system for determining integrity of an evaporative emission control space performs a leak detection test by negatively pressurizing the evaporative emission control space. Negative pressurizing refers to creating vacuum, i.e. sub-atmospheric pressure. One way of creating negative pressure uses engine manifold vacuum. With the engine running, any vent valves are closed, vacuum is drawn through the purge valve which is left open, and after vacuum has been drawn, the purge valve is closed. Loss of vacuum after closing of the purge valve is an indication of leakage. Vacuum may also be created naturally in other ways, such as when the vehicle is parked and the fuel system cools.